Particle sensor, exhaust system and method for determining particles in the exhaust gas

ABSTRACT

Embodiments of a particle sensor are provided. In one example, a particle sensor for an exhaust system comprises at least two inlet openings for an exhaust-gas flow of the exhaust system, wherein the inlet openings are of different sizes, and at least two sensor elements, wherein in each case one sensor element is arranged downstream of one inlet opening. In this way, the relative proportion of different-sized particles within the exhaust-gas flow may be determined.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 13/351,684, entitled “PARTICLE SENSOR, EXHAUST SYSTEM AND METHOD FORDETERMINING PARTICLES IN THE EXHAUST GAS,” filed on Jan. 17, 2012, whichclaims priority to German Patent Application No. 102011002936.2 filed onJan. 20, 2011, the entire contents of each of which are herebyincorporated by reference for all purposes.

FIELD

The disclosure relates to a particle sensor in an exhaust system.

BACKGROUND AND SUMMARY

To reduce the particle emissions of a diesel engine, use is made of sootparticle filters. For example, to monitor the effectiveness of saidfilters, sensors are used which measure the particle content of theexhaust gases flowing through the filter.

At present, resistance particle sensors are known in which two or moremetallic electrodes are formed, wherein the particles, in particularsoot particles, that are accumulated cause the electrodes, which engageinto one another in a comb-like manner, to be short-circuited, andtherefore, with increasing particle concentration on the sensor surface,a decreasing resistance or a decreasing impedance (or an increasingcurrent if a constant voltage is applied) can be measured between theelectrodes. The measured current or the change thereof can be correlatedwith the accumulated mass of particles and therefore also with theparticle concentration prevailing in the exhaust gas.

Sensors or systems of said type are known for example fromDE102008041809A1 and EP1873511A2/A3.

In general, the particle or solid body sensor may be used in the exhaustsystem to detect solid and also soluble fractions in the exhaust-gasflow. For this purpose, use is conventionally made of a resistanceelement whose resistance varies when substances from the exhaust gasprecipitate on the sensor element. This requires regular regeneration ofthe sensor by periodically increasing the temperature of the sensorelement in order to evaporate the accumulated material. The derivativeof the sensor signal with respect to time may be used to calculate themass throughflow of the solid or soluble materials in the exhaust gas.However, the size of the particles within the exhaust flow cannot bedetermined with this conventional type of sensor.

The inventors herein have recognized the issues with the above approachand offer a particle sensor to at least partly address them. In oneembodiment, a particle sensor for an exhaust system comprises at leasttwo inlet openings for an exhaust-gas flow of the exhaust system,wherein the at least two inlet openings are of different sizes, and atleast two sensor elements, wherein in each case one sensor element isarranged downstream of one inlet opening.

In this way, the relative portion of different sized particles withinthe exhaust may be determined. In some examples, the distribution ofparticle sizes within the exhaust may be monitored, and a change in thedistribution that exceeds a threshold may indicate a degraded combustioncondition, which may be mitigated by adjusting one or more engineoperating parameters.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of a particle sensor according tothe disclosure.

FIG. 2 shows the particle sensor of FIG. 1 arranged in a vehicle system.

FIG. 3 is a flow chart illustrating an example method of controllingcombustion stability according to an embodiment of the presentdisclosure.

DETAILED DESCRIPTION

According to a first aspect of the disclosure, a particle sensor for anexhaust system comprises at least two inlet openings for an exhaust-gasflow of the exhaust system, wherein the inlet openings are of differentsizes, and at least two sensor elements, wherein in each case one sensorelement is arranged downstream of one inlet opening.

The particle sensor permits an improved measurement of the particles inthe exhaust gas, because it is now possible for measurement to becarried out in a manner differentiated by particle size. It is thuspossible for certain particle sizes and also a size distribution to beidentified and examined. The specific resistance of a particle sensorwith regard to the accumulated particles is also dependent on the sizeof the particles. The size of the particles which pass to the sensorelement is determined by the size and/or shape of the inlet openings.The inlet openings act as a screen or a filter for sorting theparticles.

The sensor element may have a resistance element. This is the generallyused sensor type, which has the advantage of good integration inexisting systems.

The sensor element may be arranged in a duct adjoining the inletopening. Said duct or chamber conducts the exhaust-gas flow to thesensor element and may be adapted to the respective sensor elementand/or to the particle size or the size of the inlet opening.

The size of at least one inlet opening may be variable. For example, thesize may be varied by an actuating element in order to adapt themeasurement to certain situations or specific particle sizes. Saidvariation may also be carried out during ongoing operation, for examplein a manner controlled by a control processor of the exhaust-gasaftertreatment system.

The integral mass of particles of a size smaller than or the same as asmallest inlet opening may be a function of the sensor element arrangeddownstream of the smallest inlet opening. The smallest particles arecorrespondingly measured only behind, or in other words downstream, ofthe smallest inlet opening. The smallest particles also pass through thelarger openings to further sensor elements, but are not taken intoconsideration there. The measurement at only one sensor element is thesimplest implementation.

The integral mass of particles of a size smaller than or the same as anext-larger inlet opening than the smallest inlet opening and largerthan the smallest inlet opening may be a function of the sensor elementarranged downstream of the next-larger inlet opening minus the functionof the sensor element arranged downstream of the smallest inlet opening.For the next-larger particles, the sensor element arranged downstream ofthe next-larger inlet opening is used. To eliminate the fraction of thesmallest particles, the function of the smallest particles issubtracted. The integral mass, that is to say the particles depositedwithin a certain time period, of the next-larger particles can thus bedetermined. An analogous procedure is followed for further inletopenings and sensor elements for larger particles and/or intermediatesizes of particles.

According to a second aspect of the disclosure, an exhaust system of aninternal combustion engine for a motor vehicle comprises a particlesensor as described above. The same advantages and modifications asdescribed above apply.

According to a further aspect of the disclosure, a method fordetermining particles in the exhaust gas of an internal combustionengine comprises measuring particles of a first size downstream of afirst opening of a first size, and measuring particles of a second sizedownstream of a second opening of a second size.

The same advantages and modifications as described above apply. Theopenings filter or organize the particles according to their size; inthe process, the exhaust-gas flow is split up. The particles aresubsequently measured in the split-up exhaust-gas flows, such thatstatements can be made regarding the occurrence according to size andwith regard to the size distribution.

Particles of further sizes may be measured. For example, three particlesizes may be measured downstream of three openings or inlet openings. Aprecise measurement of the size distribution in the exhaust-gas flow canthus also be carried out.

The particles may be measured by a sensor element, in particular aresistance element. This is the generally routine approach, which offersgood compatibility and a broad range of uses.

The number and size of the openings may be determined by the sootdistribution in the exhaust gas. In this way, the particle sensor may beadapted to the expected composition of the exhaust-gas flow and thusoptimally set for the respective engine and/or the exhaust-gasaftertreatment system. Alternatively, it is also possible to use othercharacteristics of the exhaust gas for the definition of the particlesensor, such as for example the distribution of solid particles or offine dust.

The size of at least one opening can be varied. This increasesflexibility during operation, because an adaptation and thereforeadjustment of the measurement range is possible even after theinstallation of the particle sensor.

The size of an opening may be varied in order to examine a certainparticle size. This is a targeted setting of the particle sensor to acertain measurement size.

The size of an opening may be varied in conjunction with a cleaning of asensor element arranged downstream of the opening. During a cleaning orregeneration of the accumulated particles, the sensor element may, inthe case of soot particles, be heated by electric heating totemperatures of over 550 to 600° C., as a result of which the particlesare evaporated.

The drawings serve merely for the explanation of the disclosure, and donot restrict the disclosure. The drawings and the individual parts arenot necessarily drawn to scale. The same reference symbols are used todenote identical or similar parts.

FIG. 1 shows a particle sensor 100 for measuring particles in anexhaust-gas flow 20 of an internal combustion engine for example of amotor vehicle. The particle sensor 100 is a constituent part of anexhaust system 101 or is arranged in an exhaust system 101. Particlessuch as for example soot particles of different sizes are present in theexhaust-gas flow 20.

Aside from merely detecting the amount of particles in the exhaust-gasflow, the particle sensor 100 can also detect a size distribution of theparticles.

For this purpose, the particle sensor 100 has three inlet openings 1, 2and 3. The inlet openings are of different sizes, wherein the inletopening 1 is the smallest inlet opening, the inlet opening 2 is amid-sized inlet opening, and the inlet opening 3 is the largest inletopening. The inlet openings 1, 2 and 3 act as a filter or a screen whichassigns the particles to the respective inlet opening as a function oftheir size.

Illustrated by way of example are three size distributions D3 4, D2 5and D1 6. The size distribution 6 encompasses particles with the largestcircumference or diameter. The particles of size distribution 6 or D1can pass only through the largest inlet opening 3 but cannot passthrough the smaller inlet openings 1 or 2. The particles of sizedistribution 5 or D2 can pass through the largest inlet opening 3 andthe mid-sized inlet opening 2 but cannot pass through the smallest inletopening 1. The particles of size distribution 4 or D3 can pass throughall three inlet openings 1, 2 and 3.

The inlet opening 1 is adjoined by a duct 7 through which the smallestparticles, that is to say the particles of the size distribution D3,flow in the direction of the arrow 8. The inlet opening 2 is adjoined bya duct 9 through which the smallest and mid-sized particles, that is tosay the particles of the size distributions D2 and D3, flow in thedirection of the arrow 10. The inlet opening 3 is adjoined by a duct 11through which the smallest, mid-sized and largest particles, that is tosay the particles of the size distributions D1, D2 and D3, flow in thedirection of the arrow 12. The three inlet openings 1, 2 and 3 form aninlet side 13 of the particle sensor 100. The exhaust-gas flow 2 emergesfrom the particle sensor 100 again at an outlet side 14 of the particlesensor 100.

A first sensor element 15 is arranged in the first duct 7.Correspondingly, a second sensor element 16 is arranged in the secondduct 9. A third sensor element 17 is arranged in the third duct 11.

Each sensor element 15, 16 and 17 comprises a resistance element 18which is connected to an evaluation unit 19. For the sake of clarity,only the sensor element 15 has been provided with the reference numeralsfor the resistance element and the evaluation unit. The evaluation unit19 may be a constituent part of the sensor element, of the particlesensor 100 and/or of a controller, such as for example a controller ofthe exhaust-gas aftertreatment system.

It is also possible for two or more than three size distributions and,correspondingly, inlet openings, ducts and sensor elements to be takeninto consideration or used. The number is determined in accordance withthe desired measurement resolution and/or the characteristics of theengine or of the exhaust-gas aftertreatment system.

In the example shown in FIG. 1, three size distributions D1, D2 and D3are measured by virtue of the accumulated particles, in this case forexample soot particles, being measured using three sensor elements 15,16 and 17 (R1, R2 and R3). The size of the particles entering in eachcase is controlled by the size of the respective inlet opening 1, 2 and3.

The integral mass of the respective sizes is calculated or measured bymeasuring the variation of the respective sensor element 15, 16 and 17or of the associated resistance element 18.

In detail, the integral masses are determined as follows:

The integral mass of particles of a size D smaller than or equal to D3is measured as a function of the sensor element 15 (Func(R3)).

The integral mass of particles of a size D larger than D3 and smallerthan or equal to D2 is measured as a function of the sensor element 16minus the function of the sensor element 15 (Func(R2)−Func(R3)). Thesubtraction of the function of the sensor element 15 serves for thecorrection of the measurement values. The effects caused by theparticles of size D smaller than or equal to D3 are thus eliminated.

The integral mass of particles of a size D larger than D2 and smallerthan or equal to D1 is measured as a function of the sensor element 17minus the function of the sensor element 16 and minus the function ofthe sensor element 15 (Func(R1)−Func(R2)−Func(R3)). The subtraction ofthe functions of the sensor elements 16 and 15 serves for the correctionof the measurement values. The effects caused by the particles of size Dsmaller than or equal to D2 are thus eliminated.

The number and size of the inlet openings or of the sensor elements maybe selected on the basis of the required soot distribution, in orderthereby to have an optimum measurement and monitoring in the relevantsize ranges. The calculation for more than three particle sizedistributions takes place analogously to the determination describedabove.

Furthermore, one or more inlet openings may be of variable size. Theinlet size may be varied by means of a controlled actuation. It is thuspossible for a certain size distribution of interest to be set andmeasured. Measurements with a varying opening size may also be carriedout in temporal succession. The measurement resolution can thus beincreased by means of temporal extension. The measurement resolution canbe correspondingly adapted by means of the number of individualmeasurement paths (inlet opening, sensor element) or by means of one ormore variable inlet openings with temporally staggered measurements.

Said measurements may for example be carried out between cleaningprocesses of the sensor elements.

FIG. 2 shows a schematic depiction of a vehicle system 60. The vehiclesystem 60 includes an engine system 80 coupled to a fuel system 118. Theengine system 80 may include an engine 110 having a plurality ofcylinders 30. The engine 110 includes an engine intake 23 and an engineexhaust system 101. The engine intake 23 includes a throttle 62 fluidlycoupled to the engine intake manifold 44 via an intake passage 42. Theengine exhaust 101 includes an exhaust manifold 48 leading to an exhaustpassage 35 that routes exhaust gas to the atmosphere. The engine exhaust25 may include one or more emission control devices 70, which may bemounted in a close-coupled position in the exhaust. One or more emissioncontrol devices may include a three-way catalyst, lean NOx trap, dieselparticulate filter, oxidation catalyst, etc. It will be appreciated thatother components may be included in the engine such as a variety ofvalves and sensors.

Fuel system 118 may include a fuel tank 120 coupled to a fuel pumpsystem 21. The fuel pump system 21 may include one or more pumps forpressurizing fuel delivered to the injectors of engine 110, such as theexample injector 66 shown. While only a single injector 66 is shown,additional injectors are provided for each cylinder. It will beappreciated that fuel system 118 may be a return-less fuel system, areturn fuel system, or various other types of fuel system.

An EGR system 24 may be present to divert a portion of the exhaust backto the intake via EGR passage 28. Control of flow through the EGR system24 may be provided by EGR valve 22, herein positioned at the junction ofthe EGR passage 28 with the exhaust system. EGR reduces oxygen contentin the intake, which may result in lowered peak combustion temperatures,improving emissions.

FIG. 2 also shows the particle sensor 100 arranged downstream of theemissions control device 70. The sensor 100 and device 70 are arrangedin the exhaust system 101. The exhaust gas from the engine 110 may flowthrough the particle sensor 100 and out to another exhaust passage or tothe atmosphere, as shown by the arrows in FIG. 2.

The vehicle system 60 may further include control system 114. Controlsystem 114 is shown receiving information from a plurality of sensors116 (various examples of which are described herein) and sending controlsignals to a plurality of actuators 81 (various examples of which aredescribed herein). As one example, sensors 116 may include exhaust gassensor 126 located upstream of the emission control device, particlesensor 100, temperature sensor 128, and pressure sensor 129. Othersensors such as pressure, temperature, air/fuel ratio, and compositionsensors may be coupled to various locations in the vehicle system 60. Asanother example, the actuators may include fuel injector 66, valve 22,and throttle 62. The control system 114 may include a controller 112.The controller may receive input data from the various sensors, processthe input data, and trigger the actuators in response to the processedinput data based on instruction or code programmed therein correspondingto one or more routines. Example control routines are described hereinwith regard to FIG. 3.

FIG. 3 depicts a method 300 for maintaining combustion stability in anengine. Method 300 may be carried by a control system of a vehicle, suchas controller 112, in response to feedback from one or more sensors,such as particle sensor 100. Method 300 includes, at 302, determiningengine operating conditions. Engine operating conditions may includeengine speed, load, temperature, etc. Engine operating conditions mayalso include fuel injection timing and amount, whether the EGR system isenabled, and whether any emissions control devices, such as particulatefilters, are in a regeneration state. At 304, the amount of first-sizedparticles, second-sized particles, and third-sized particles in theexhaust are determined by the particle sensor. This may include routingexhaust through three inlets of the particle sensor. As explained above,each inlet of the particle sensor may be different sized such that onlythe first-sized particles may enter a first inlet, only first and secondsized particles may enter a second inlet, and first, second and thirdsized particles may enter a third inlet. In this way, the amount of eachsized particle in the exhaust may be determined.

At 306, it is determined if engine temperature is above a threshold andif the particulate filter is in a non-regeneration state. If one or bothof these conditions is not met (e.g., the answer is no) method 300returns. If the conditions are met, the particle size distribution basedon feedback from the particle sensor may be used to monitor combustionstability. As particles in the exhaust are a by-product of thecombustion process, the amount and distribution of different sizedparticles in the exhaust may indicate altered or degraded combustion. Inone example, if the portion of large particles in the exhaust increases,it may be indicative of misfire, pre-ignition, too-lean combustion,etc., which may result in increased emissions. To avoid and/or mitigatethis, at 308, it is determined if the amount and/or distribution of thedifferent sized particles in the exhaust has changed more than athreshold amount. A baseline amount and distribution of different sizedparticles in the exhaust may be determined based on sensor readingsduring stable combustion immediately following engine manufacture, forexample, or based on a model determined off-line, or based on a rollingaverage of sensor readings during the current engine operation. If theamount or distribution of the particles deviates significantly from thisbaseline, it may indicate a degraded combustion condition, or degradedparticulate filter condition.

The first, second, and third sized particles may be the smallest thirdof all particles, the intermediate third of all particles, and thelargest third of all particles, respectively. However, the first,second, and third sized particles may be other suitable sizeclassifications. For example, a change in the amount of very large-sizedparticles may be more indicative of degraded combustion than a change inthe amount of other sized particles. In this case, the first sizedparticles may the smallest 80% of all sized particles, while the thirdsized particles may be the largest 10% of all sized particles, in orderto more accurately monitor a change in the amount of the verylarge-sized particles.

In one example, a change in the distribution of different sizedparticles above a threshold may include a change in one or more sizedparticles of 10%, or 20%, or another suitable amount, of the totalparticles in the exhaust. In other example, the distribution may notchange, but the amount of all the total particles may change.

If the amount and/or distribution of particles have not changed morethan the threshold amount, method 300 returns. If the amount and/ordistribution have changed by more than the threshold amount, a degradedcombustion condition is indicated at 310. To mitigate this, at 312, fuelinjection and/or an EGR valve may be adjusted to improve combustionstability. In one example, if the largest-sized particles increase bymore than 10%, the EGR valve may be adjusted to lower the EGR percentagein the charge air, thus increasing the amount of oxygen to promoteincreased combustion stability. In another example, if the largest-sizedparticles increase by more than 10%, the EGR valve may be adjusted toincrease the EGR percentage to lower peak combustion temperatures. Fuelinjection may additionally or alternatively be adjusted. If the amountof the smallest-sized particles increases by more than 10%, for example,it may be desired to increase the amount of large-sized particles, asthe large-sized particles may be better retained in the particulatefilter than the small sized particles. To increase particle size, fuelmay be injected later in some embodiments, or may be injected earlier inother embodiments.

Thus, method 300 provides for monitoring the distribution of two or moresized particles in the exhaust system of a vehicle. By monitoring achange in the amount and/or distribution of each sized particle,instability in combustion may be determined. In some embodiments, themonitoring of change in particle size may only be performed when enginetemperature is above a threshold, such as 100° F., so that combustionconditions under standard, non-cold operation can be monitored. Further,the degraded combustion may not be indicated if the particulate filteris undergoing a regeneration, as the regeneration may result in changesin particulate size that are not indicative of combustion instability ora degraded combustion condition.

Thus, in one embodiment, FIG. 3 provides a method for maintainingcombustion stability in an engine comprising monitoring a proportion ofeach of first, second, and third-sized particulate matter within anexhaust stream via a particle sensor, and during select conditions,adjusting fuel injection and/or an exhaust gas recirculation rate inresponse to a change in a distribution of the first, second, andthird-sized particles exceeding a threshold. The method may also includemonitoring the proportion of each of first, second and third-sizedparticulate matter within the exhaust stream via the particle sensorfurther comprises routing the exhaust stream through a small-sized inletopening, a medium-sized inlet opening, and a large-sized inlet openingof the particle sensor, wherein each inlet opening is an inlet openingfor a duct housing a sensing element. The select conditions comprise aparticulate filter upstream of the particle sensor being in anon-regeneration state, and/or engine temperature being above athreshold.

FIG. 3 may also provide an engine method comprising adjusting engineoperation in response to sizes of exhaust particulates. The method mayalso include that the adjusting includes adjusting fuel injection andexhaust gas recirculation in response to a change in a relativeproportion of different sized exhaust particulates.

It will be appreciated that the configurations and methods disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

The invention claimed is:
 1. A particle sensor for an exhaust system,comprising: a first inlet opening and a second inlet opening forreceiving an exhaust-gas flow of the exhaust system, wherein the firstinlet opening and the second inlet opening are of different sizes; and afirst sensor element and a second sensor element, wherein the firstsensor element is arranged downstream of the first inlet opening fordetecting a first amount of particles flown through the first inletopening, and the second sensor element is arranged downstream of thesecond inlet opening for detecting a second amount of particles flownthrough the second inlet opening.
 2. The particle sensor as claimed inclaim 1, wherein each of the first sensor element and the second sensorelement has a resistance element.
 3. The particle sensor as claimed inclaim 1, wherein each of the first sensor element and the second sensorelement is arranged in a duct which adjoins a respective inlet opening,wherein different sensor elements are arranged in different ducts. 4.The particle sensor as claimed in claim 1, wherein a size of at leastone of the first inlet opening and the second inlet opening is variable.5. The particle sensor as claimed in claim 1, wherein the first inletopening is a smallest inlet opening, and wherein an integral mass ofparticles of a size smaller than or the same as the smallest inletopening is a function of measurements by the first sensor elementarranged downstream of the smallest inlet opening.
 6. The particlesensor as claimed in claim 5, wherein the second inlet opening is anext-larger inlet opening than the smallest inlet opening, and whereinan integral mass of particles of a size smaller than or the same as thenext-larger inlet opening is a function of measurements by the secondsensor element arranged downstream of the next-larger inlet openingminus the function of measurements by the first sensor element arrangeddownstream of the smallest inlet opening.
 7. A method for detectingparticles in an exhaust gas of an internal combustion engine,comprising: flowing the exhaust gas through a particle filter with afirst opening of a first opening size and a second opening of a secondopening size, the first opening size different from the second openingsize; measuring a first amount of the particles of a first particle sizedownstream of the first opening of a first size via a first sensorelement; and measuring a second amount of the particles of a secondparticle size downstream of the second opening via a second sensorelement, different from the first sensor element, wherein the secondparticle size is different from the first particle size.
 8. The methodas claimed in claim 7, wherein particles of sizes different from thefirst particle size and the second particle size are measured.
 9. Themethod as claimed in claim 7, wherein the particles are measured by aresistance element of each of the first sensor element and the secondsensor element.
 10. The method as claimed in claim 7, further comprisingflowing particles through additional openings of the particle filter,and wherein a number of the additional openings, the first opening size,the second opening size, and opening sizes of the additional openingsare determined by a soot distribution in the exhaust gas.
 11. The methodas claimed in claim 7, wherein at least one of the first opening sizeand the second opening size can be varied.
 12. The method as claimed inclaim 11, wherein the at least one of the first opening size and thesecond opening size is varied based on particle size.
 13. The method asclaimed in claim 11, wherein the at least one of the first opening sizeand the second opening size is varied conjunction with a cleaning of asensor element arranged downstream of each of the openings.